1. Field of the Invention
In the typical magnetic disc recording system, a plurality of recording surfaces are employed, each being accessed by one or more transducing heads associated with that surface only. The transducing heads are all mounted on head arms in turn fixed to a carriage. By shifting the carriage radially* with respect to the discs, the transducing heads may be positioned adjacent any of the recording tracks thereon. A linear motor is used to properly position the carriage to permit data tracks specified by control signals to be accessed by the heads. The means by which the actual radial positions of the various tracks are determined has been accomplished in a variety of ways. An optical grating scribed with lines corresponding to the desired track positions can be employed. Typical such gratings are described in U.S. Pat. Application Ser. No. 454,350, now U.S. Pat. No. 3,872,575, filed Mar. 25, 1974 (Lee et al) and Ser. No. 454,351, now U.S. Pat. No. 3,871,064 filed Mar. 25, 1974 (Schraeder). Specially designed magnetic coils can be used to produce a magnetic signal pattern corresponding to the difference between adjacent disc radii. See U.S. Pat. 3,720,930 (Elsing). In the preferred design employed today, one recording surface contains only servo tracks which are continually read, providing signals specifying the carriage position. See U.S. Pat. Application Ser. No. 458,262, now U.S. Pat. No. 3,903,545, filed Apr. 5, 1974 (Beecroft I). A problem common to all these devices is maintaining the magnetic transducing heads in precise radial relationship with respect to each other and to the carriage after the position of each individual head has been initially adjusted to the desired, nominal position vis-a-vis all the other heads and the carriage. FNT * The term "radial" or "radially" hereafter refers to the recording disc radius.
In the designs preferred today, each transducing head is embedded in a pad which during operation rides on a thin (5-200 micro-inches) air film between the pad's bearing surface and the disc. The characteristics of the air film are such that the natural tendency of the pad is to ride a greater distance from the disc than is desired. It is therefore necessary to force the pad to the desired distance from the disc. This is accomplished by applying force on a load surface of the pad which faces away from the bearing surface. Because of recording surface movement caused by spindle bearing runout and lack of disc flatness, each individual head must be capable of shifting very rapidly with respect to its supporting arm, perpendicular to its recording surface. Therefore, the force is preferably applied by a spring whose spring constant is relatively constant for the head motion involved, to permit the head to maintain a constant distance from the recording surface. Because capability of the pads to shift very rapidly with respect to their supporting structures in response to these recording surface permits the pads to ride at a desirable small distance from the recording surfaces, it is preferred to make the high mass head arm itself very rigid and allow only the low mass head and its immediate support structure to follow the movements of the recording surface. See. U.S. Pat. Application Ser. No. 526,554, now U.S, Pat. No.3,914,792, filed Nov. 25, 1974 (Beecroft II).
When retracting the head arms, it is necessary to lift the pads away from the recording surface before the air film disappears at the rim of the disc. To simplify this procedure, the lifting force is applied to an intermediate arm which is positioned between the spring arm and the pad, and which also transmits the spring force counteracting the air film lift to the pad. This intermediate arm is pivoted on a shaft attached to the head arm. Because the hole in the intermediate arm through which this shaft passes must be slightly larger than the shaft itself to prevent binding, the intermediate arm can during operation shift radially. This shift, or backlash, is very small, on the order of a few ten thousandths of an inch at most. Fast accelerations and decelerations caused by vibration and carriage motion, and accidental rotation of the intermediate arm during installation all can cause this slight shifting of the intermediate arm. Errors so introduced though small are frictionally transmitted to the pad and form a significant percentage of the total tolerance allowable for accurate accessing of individual data tracks.
2. Description of the Prior Art
The Beecroft II application involves a flexible coupling between the spring arm and the head itself which reduces radial stress on the head pad support. Merely decreasing the tolerance between the pivot shaft for the intermediate arm and the holes through which it passes runs the risk of causing binding and sticking of the intermediate arm. Applicant is not aware of any head arm mechanisms which self-correct pivot backlash.